Evidence of zinc deficiency in several aged populations lends urgency to the need for studies designed to evaluate how aging may alter zinc homestasis and to identify those factors responsible for the development of the deficiency. So little is presently known about trace element metabolism in the aged individual that it is not possible to provide sufficient basic information needed by those making dietary recommendations or responsible for the health care of the aged. This research is designed to provide a greater understanding of the ways in which aging influences zinc metabolism and of the response of the aged animal to provision of suboptimal levels of dietary zinc. Animals of various ages ranging from young to aged (2 to 26 months) will be fed diets containing levels of zinc from severely deficient to generously adequate (2 to 50 mg Zn/kg diet) for two weeks prior to a 14 day balance study. The radioisotope dilution technique in conjunction with the balance study will be used to establish levels of true absorption (i.e. apparent absorption corrected for fecal excretion of zinc of endogenous origin) and of true retention in animals of various ages and exposed to the various dietary treatments. Compositional studies and determination of the amount and specific activity of zinc in plasma and various tissues will be used to evaluate the changes that occur in the distribution of this essential trace element as a function of aging, in response to changes in dietary zinc intake, and in response to changed in body composition.